1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly to, an image forming apparatus that has amorphous silicon photosensitive members.
2. Related Background Art
Recently, there have widely accepted in the market such composite apparatuses that are provided with all the output terminals such as a copy machine, a printer, a facsimile etc. That is, those electrophotographic apparatuses have been widely employed as network accommodating output terminals. In the utilization of those output terminals, their duty cycle is thought of as an important parameter. The duty cycle, which refers to the limit number of sheets over which the body can continue to work without maintenance, has a life of the photosensitive drum as its largest rate-determining factor. In addition, from the viewpoint of ecology, it has become an important object to eliminate waste as much as possible or to reduce consumables, to elongate their life, and to improve their reliabilities. Moreover, with conventional analog devices largely having been converted into digital ones, it is now required to provide an analog-device equivalent costs or less.
With the above as a background, as image carrying members, amorphous silicon photosensitive members have largely been employed gradually as being indispensable in particular in the high-speed machines, which require high reliabilities, because of their large hardness (1000 kg/m.sup.2 or higher of JIS-standard Vickers hardness), high durability, high heat resistance, and excellent ecological stability.
In such apparatuses, however, not only toner but also minute paper particles occurring from paper utilized mostly as a transfer material and resultantly-separated organic substances, and corona products and other foreign matter created by the existence of the built-in high-tension members will adhere to the surfaces of the image carrying member and adversely affect the picture quality; in high-humidity environments, moreover, such foreign matter may possible reduce the resistance to prevent the formation of clear electrostatic latent images, thus deteriorating the picture quality.
It is known that such picture-quality deteriorating phenomena are likely to occur with amorphous silicon photosensitive members, which form films due to glow discharge decomposition of silane and the like substances. To avoid such problems, especially when one-component magnetic toner is used, it is proposed that in a cleaning apparatus, as viewed in the travel direction of the image forming member, a magnet roller should be arranged on the upstream side of the cleaning blade to make a magnetic brush of some of toner collected into the cleaning apparatus, which brush be in turn brought in contact with the surface of the image forming member to re-supply magnetic toner so that the toner particles on the blade side may, by their abrasive action, remove the above-mentioned various kinds of foreign matter by sliding operation. Such a method will have a less local unevenness of the abrasive action on the image carrying member surface and a smaller deterioration of that surface than such an approach that a web or rubber roller polish, by sliding, that surface with abrasives. According to the above-mentioned method, with a heater being provided on the image carrying member, an attendant method may be employed that lowers the surrounding humidity even during stand-by operation at night, so as to prevent the resistance of the surface of the image carrying member from decreasing, thus blocking to some extent the deterioration in the picture quality due to the earlier mentioned factors.
In an image forming apparatus that repeats a process of transferring onto a transfer material mainly made of paper a transferable toner image formed on the image carrying member surface, the residual toner on the image carrying member needs to be removed for each process without shifting it onto the transfer material.
To this end, among many proposals made so far, such a cleaning method is widely used that a cleaning blade made of urethane rubber or other elastic materials is used to dust away the above-mentioned residual toner, because it has a simple and compact configuration with lower costs and is excellent in performance of removing toner. As the rubber material for the cleaning blade, urethane rubber is generally used for its high hardness, good elasticity, good wear-resistance, high mechanical strength, good oil-resistance, high ozone-resistance etc.
However, from a viewpoint of further energy saving requirements in recent years, there is a strong need to eliminate the heater provided on the image carrying member.
Possible factors for image smearing include toner, minute paper particles generated from paper used mostly as a transfer material, the resultantly deposited organic substances, and such components as nitric ions given as a result of oxidation of nitrogen in the air at the same time as the generation of various kinds of metal oxides and oxidized compounds generated at the corona discharge with high energy from build-in high tension members, which all attach to the surface of the image carrying member as it is used to form thin films (hereinafter called filming membrane) on the photosensitive member surface, thus absorbing the humidity in the high humidity environments to lower the resistance and prevent the formation of clear electrostatic latent images, which leads to the deterioration of the picture quality.
The above-mentioned filming membrane layer has been confirmed to measure in thickness about 30 to 80 .ANG. by an optical method in our experiments. In our experiments of durability test conducted in this case, however, it has been found that the above-mentioned filming membrane initially measured in thickness about 30 to 80 .ANG. and then changed little but as time passes, the image deterioration, which could have initially been eliminated by dry-wiping, water-wiping, or alcohol-wiping, cannot be done so. It has been found that in some cases the drum surface which has undergone the durability test to some extent in such a state cannot sufficiently be freed from image deterioration, unless it is polished with abrasive grains of 0.3 to 2.0 .mu.m cerium oxide (CeO.sub.2) dispersed in alcohol. This phenomenon is likely to occur especially when no drum heater is provided. With further discussion, we observed the surface of a photosensitive member having a variety of surface geometry both in the initial state and after the durability test using an atomic force microscope (AFM). After the durability test, the photosensitive member surface appeared to be almost flat as a result of wear as compared to that in the initial state. We conducted heating or ultra-sonic cleansing on the photosensitive member surface after the durability test, using an organic solvent (MEK, peroxodisulfuric sodium (Na.sub.2 S.sub.2 O.sub.8)). Then, it was found that the amount of the filming at in particular the recess varies with the initial average gradient .DELTA.a of the photosensitive member and, there is a corresponding correlation in the occurrence of the image smearing. As mentioned above, when no drum heater is installed, it is important to devise an image forming apparatus that forms no filming membranes from the initial state by use of the image carrying member and, among them, to provide the drum surface with the above-mentioned function.
Second, our experiments have shown that as the image carrying member is used on, the friction between the drum and the post-transfer residual toner given by the cleaning blade is increased.
This is considered to have been caused by a fact that the filming membrane increases the contact degree between the cleaning blade and the drum surface and also that between the post-transfer residual toner and the drum surface, thus increasing the friction.
An increase in friction is considered to increase the shearing stress of the cleaning blade, that among toner, and that in the vicinity of the drum surface. As a result, this is considered to lead to the chipping of the cleaning blade, the occurrence due to increases in the amount of heat generated by increases in the permanent strain shearing stress and also increases in the fatigue wear due in increases in the intra-drum stress.
Third, recently the image forming apparatus has not only been used as a copy machine but also as a printer widely. In addition, the apparatus has been provided with such application functions as feeding functions and sorting functions, so that its one job can continuously process 4000 sheets or more of paper. This means, for example, it is just estimated that an apparatus for 50 sheets/A4 size can operate for 80 minutes or more. In such a situation, the ambient temperature is considered to rise up to near 50.degree. C. near the photosensitive member and higher at the butting (nipping) part between the cleaning blade and the photosensitive member. Therefore, it is considered that melt-adhesion frequently occurs on the photosensitive member.
Fourth, the cleaning blade has been determined in terms of its cleaning latitude by the butting angle against the drum surface, the free length, the thickness, the total pressure, the linear pressure, and the properties of rubber used as the cleaning blade. For example, Japanese Patent Application Laid-Open No. 6-274079 describes that in order to eliminate the chipping on the cleaning blade and the cleaning blade fluttering in the low-temperature and low-humidity environments, the peak temperature of tans should be at -13.degree. C. to -16.degree. C., the impact resilience should be higher, and the Young's modulus should be lower. However, it describes nothing about the impact resilience under the high-temperature region.
Our experiments conducted recently have shown that the above-mentioned toner melt-adhesion can be suppressed by the modulus of repulsion elasticity of the rubber material used as the cleaning blade.
Also, the causes of toner melt-adhesion on the drum is different between an amorphous silicon photosensitive member and an organic photosensitive member. In the case of an organic photosensitive member, toner melt-adhesion occurs because minute particles of an external additive such as silica are embedded into the photosensitive member surface to provide nucleuses, whereas in the case of an amorphous silicon photosensitive member, raining-state toner melt-adhesion occurs when there are few-.mu.m-height protrusions on the photosensitive member surface or even when there are nothing that provide nucleuses.
However, rubber materials are largely temperature dependent and, urethane rubber used as the cleaning blade is particularly temperature-dependent.